Cross-point switches are commonly used in optical communication systems, test equipment, and transreceivers. Typical cross-point switch architecture is shown in FIG. 1. More particularly, high-frequency cross-point switches are commonly used in optical-millimeter wave-optical (OMO) switches. In OMO switches, the optical signal is first converted to a millimeter-wave (mm-wave) baseband signal. Then, switching is performed according to the requirements. Finally, the signal is converted to an optical wavelength again by modulating a laser diode. This scheme allows reshaping, retiming, and regenerating of signals easily because these functions are implemented in the electrical domain.
One critical requirement of this scheme is its very wide bandwidth. For a 40 GBit/s data rate, the cross-point switch must have a 3-dB bandwidth of at least 0.1 GHz to 25 GHz. After 25 GHz, the band should roll off smoothly.
Another important point concerns the distribution of bias lines to switching elements. Although for small switch sizes (2×2, 4×4) this can be manageable, for relatively large matrix sizes (16×16, 16×32) it can be extremely difficult or nearly impossible to distribute all of the biasing lines. Bias lines are used to activate and deactivate individual switching elements. In addition to this, DC power must also be supplied to switching elements if they consist of active elements.
Switch loss is a third important consideration. High-frequency passive cross-point switches that have a large number of RF inputs and outputs usually have high insertion losses. The high insertion loss stems from the fact that the transmission lines that form the matrix must be terminated, as with resistors, 10, to eliminate reflections that deteriorate the pulse shape (see FIG. 1). Provided that all of the lines have the same characteristic impedance and are terminated with the same impedance, this results in a minimum of 6 dB theoretical insertion loss for a high-frequency cross-point switch. Any losses due to signal transitions, metallization and lossy dielectrics would be on top of this figure. However, the absolute value of insertion loss is not the primary issue for optical switches as long as it remains above a critical level because the system has 3R (regenerate-reshape-retiming) functionality at some level. Therefore, 7–10 dB insertion losses are acceptable as long as the on/off insertion loss ratio is greater than approximately 30 dB at the highest operating frequency. On the other hand, although 7–10 dB insertion loss per switch matrix may be manageable, cascading such matrix elements to achieve higher port count matrices can become troublesome without inserting intermediate amplifier stages to boost up the signal level.
Although the absolute value of insertion loss is therefore not the paramount consideration for an optical switch in most of the cases, the coupling between the channels is. Therefore, the switching fabric must be designed to minimize channel-to-channel coupling.
A switch matrix using latching PIN diodes based on a GaAs process can address all of these issues successfully to some extent. Perhaps the main advantage of latching PIN diodes is the possibility of using RF lines to carry the switching signals (i.e., x-y addressing). This greatly reduces the requirements for bias lines. For instance, for a 16×16 switch, one would require 256 bias lines if it was attempted to bias each junction individually. However, if one uses latching diodes, then one would need only 32 bias lines, which can be the same as the RF lines. Latching PIN diodes have been employed in low frequency networks (i.e., telephony) for a long time. Employing a mm-wave latching PIN diode in a cross-point switch architecture has occurred relatively recently. Despite their advantages over conventional PIN diode switch matrices, GaAs latching PIN diode matrices have the following drawbacks: large circuit size, relatively high cost (i.e., low yield), difficulty in incorporating on-chip amplifiers, and difficulty in incorporating digital circuits.